1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens. The present invention is suitable for, for example, an image pickup apparatus, such as a video camera, an electronic still camera, a broadcast camera, or a surveillance camera, which includes a solid-state image pickup element, or a silver-halide film camera.
2. Description of the Related Art
Recently, image pickup apparatuses have become smaller with increased functionality. Accordingly, demand has increased for small, high-zoom-ratio (high-magnification-variation-ratio) zoom lenses having a small length and high resolution for use in imaging optical systems of the image pickup apparatuses.
To comply with such a demand, a rear-focus zoom lens has been proposed which performs focusing by moving lens units other than a first lens unit disposed at the object side.
In the rear-focus zoom lens, an effective diameter of the first lens unit is generally small compared to that in a zoom lens in which the first lens unit is moved during focusing. Therefore, the size of the entire lens system can be easily reduced.
In addition, a macro imaging operation can be easily performed, and the required driving force can be reduced since small, light lens units are moved. Therefore, quick focusing can be performed.
A four-unit zoom lens including four lens units, which are a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power arranged in order from the object side to the image side, is known as the rear-focus zoom lens.
In the four-unit zoom lens, zooming is performed by moving the second lens unit, and focusing and compensation for image plane variation caused during zooming are performed by moving the fourth lens unit (see U.S. Pat. Nos. 5,963,378 and 6,166,864).
In addition, a zoom lens which performs zooming by moving each lens unit and focusing by moving the fourth lens unit is also known (see U.S. Pat. No. 7,190,529 and Japanese Patent Laid-Open No. 2007-212537).
In addition, U.S. Pat. No. 7,206,137 describes a four-unit or five-unit zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, which are arranged in order from the object side to the image side. This zoom lens performs zooming by moving the first to fourth lens units and focusing by moving the fourth lens unit.
In a five-unit zoom lens according to U.S. Pat. No. 7,206,137, a fifth lens unit is fixed during zooming.
In general, the zoom ratio can be increased and the size of the zoom lens can be reduced by increasing the refractive power of each lens unit included in the zoom lens and reducing the number of lenses.
However, as the refractive power of each lens surface increases, the lens thickness increases and the length of the lens system cannot be sufficiently reduced. In addition, it becomes difficult to correct aberrations, such as chromatic aberrations, at the telephoto end.
In addition, as the zoom ratio increases, the influence of assembly errors, such as tilting of each lens or lens unit, increases.
If the sensitivity to eccentricity of each lens or lens unit is high, the optical performance is largely reduced due to the eccentricity caused by assembly errors. As a result, the optical performance is also largely reduced in a vibration reducing state.
Therefore, in the zoom lens, the sensitivity to eccentricity of each lens or lens unit is set as low as possible.
In the above-described four-unit zoom lens or five-unit zoom lens, to achieve an increase in the zoom ratio and reduction in the size of the entire lens system while maintaining good optical performance, it is important to adequately set the refractive power of each lens unit, the lens structure, and the moving condition of each lens unit during zooming.
In particular, in order to effectively reduce the size of the entire lens system, it is important to reduce not only the overall length of the lens system but also a front lens diameter and the effective diameter of each of the lenses included in the first lens unit.
In general, lenses included in the first lens unit have a larger effective diameter than those of the lenses included in other lens units. Therefore, if the effective diameter of the first lens unit can be reduced, the lens thickness required to obtain a lens edge portion can also be reduced. As a result, the overall length of the lens system can be reduced.
In the case where the size of the entire lens system of the above-described four-unit or five-unit zoom lens is reduced, the effective diameter of the first lens unit is determined by the light-ray heights at a plurality of zoom positions.
For example, in the case where the first lens unit includes a first lens, a second lens, and a third lens arranged in order from the object side to the image side, the effective diameter of the first lens is determined by a marginal light ray height at the wide-angle end. In addition, the effective diameter of the second lens is determined by a marginal light ray height in an intermediate zoom range. The effective diameter of the third lens is determined by a marginal light ray height at the telephoto end.
Therefore, to reduce the size of the entire lens system, it is important to adequately set not only the refractive power of each lens unit but also the locus thereof during zooming in a zoom range including the intermediate zoom range.